1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same. In particular, the present invention relates to a semiconductor device that uses phase change storage elements, and a method of manufacturing the same.
2. Description of Related Art
Phase change random access memories (PRAMS) have been known which comprise storage elements each including a phase change material and an upper electrode and a lower electrode (heater electrodes) that are arranged with the phase change material therebetween. Hereinafter, the storage elements of the PRAMs may be referred to as “phase change storage elements”. Patent Document 1 (Japanese Patent Application Laid-Open No. 2008-071797) and Patent Document 2 (Japanese Patent Application Laid-Open No. 2008-311664) disclose examples of such a PRAM. In PRAMs, writing on a phase change storage element is performed by passing a write current between upper and lower electrodes and changing a phase of the phase change material with the resulting heat.
The upper electrode is a conductor that constitutes bit lines and is arranged to have an electrical connection with the top surface of the phase change material. The lower electrode is a conductor of pillar shape. As indicated in Patent Documents 1 and 2, the lower electrode is arranged so that its top surface has a close contact with the bottom surface of the phase change material. The bottom surface of the lower electrode is electrically connected to one of two diffusion layers of a cell transistor through a contact plug. The other diffusion layer of the cell transistor is connected to ground wiring. Gate electrodes of the cell transistors constitute word lines. With the foregoing configuration, a PRAM can write and read a phase change storage element at the intersection of a selected word line and a selected bit line.
To increase current density for improved heating efficiency, the contact area between each of the heater electrode (an upper electrode or an lower electrode) and the phase change material is preferably small. The heater electrode and the phase change material are typically formed by stacking them in order inside a through hole formed in an insulation film. According to such a method, the lower limit of the contact areas is determined by the minimum feature size. Therefore, techniques realizing a smaller contact area than the lower limit have been desired.
FIG. 12 of Patent Document 1 and FIG. 10 of Patent Document 2 disclose examples of such techniques. In the examples, a heater electrode is formed in a ring shape and a phase change material is formed thereon in a similar ring shape, thereby reducing the contact area. Furthermore, a technique realizing a reduction of the contact areas by forming a sidewall insulation film inside through holes and thus reducing the diameter of the through holes is under study. However, the foregoing techniques are far from achieving a sufficient reduction in contact area. A technology capable of a further reduction in contact area has thus been desired.